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National Instruments Corporation
5-1
SCXI-1141/1142/1143 User Manual
5
Calibrating the
SCXI-1141/1142/1143 Module
The onboard calibration hardware that calibrates the
SCXI-1141/1142/1143 module consists of an EEPROM to store the gain
calibration constants, calibration circuitry on the amplifier inputs, and a
potentiometer to adjust the AC gain of the filter.
Calibration is a method for nulling offset-error sources that reduce
measurement accuracy. Before calibrating, disconnect all input signals
from the amplifier inputs and connect the inputs to ground. Calibration
determines each output of the SCXI-1141/1142/1143 module at a given
gain of the amplifier. You should calibrate at the start of an application for
each gain to be used. Doing this helps eliminate error due to drift in the
amplifier internal circuitry and increases the accuracy of the measurement.
The calibration path is different from the analog input path. Therefore, even
after calibration, a residual input offset still exists and has a value of less
than 6 µV. The offsets of the filters are compensated for internally;
generally only the offsets of the amplifiers need to be calibrated. The filters
should be in bypass mode during calibration to avoid long settling times. If
you are calibrating the offsets of both the amplifiers and the filters, the
filters must not be in bypass mode and the filters should be given 15 s to
completely settle to the calibration voltage.
The DC gain errors of the filters (below 0.1 Hz) are also compensated for
internally. You can adjust the AC gain of each filter with a trimming
potentiometer that has an adjustment range of ±0.15 dB. The gain errors of
the amplifiers are the same for DC and AC signals. Calibration constants
representing these errors for each gain of each amplifier are stored in the
onboard EEPROM for future use and for automatic calibration.
The AC gain of the filters is calibrated at the factory, and the calibration
constants for the amplifiers are already loaded into the EEPROM. When
you need to recalibrate SCXI-1141/1142/1143 modules download the
SCXI-1141/1142/1143 Calibration Procedure
from
ni.com/calibration
.
